Electrothermal atomization means for analytical spectrometry

ABSTRACT

An electrically heatable hollow-body furnace with a secondary surface on which an analyte of a sample can be condensed prior to being atomized. The furnace is constructed in two sections which are capable of being electrically heated independently of one another. The secondary surface is defined by a surface of one of the sections. A process for atomizing an analyte of a sample to be examined, utilizing the device according to the following steps: introducing the sample into a hollow-body furnace having a secondary surface, condensing the analyte on the secondary surface and atomizing the analyte.

This application is a Divisional application of U.S. patent applicationSer. No. 08/792,322, filed on Jan. 31, 1997, now U.S. Pat. No.5,866,431.

FIELD OF THE INVENTION

The present invention refers to an atomization means for converting ananalyte of a sample to be examined into the atomized state, saidatomization means comprising an electrically heatable hollow-bodyfurnace including a secondary surface on which an analyte of a sample tobe examined can be condensed prior to being converted into the atomizedstate for examination.

Furthermore, the present invention refers to a process for atomizing ananalyte of a sample to be examined, said process comprising thefollowing steps: introducing the sample into a hollow-body furnacehaving a secondary surface, condensing the analyte on said secondarysurface and atomizing the analyte.

BACKGROUND ART

Such a device and such a process are known from the publication"Electrothermal atomic absorption spectrometry by reatomization from asecond trapping surface" by P. Hocquellet in Spectrochimica Acta, 47B,pages 719-729, 1992, and also from T. M. Rettberg and J. A. Holcombe,Spectrochimica Acta, 41B, pages 377-389, 1986.

The known atomization means is shown in FIG. 4. It comprises a heatabletubular furnace 1 which consists of graphite and which has providedtherein an additional element 60 of graphite having an arched surface50. An analyte of a sample to be examined can be condensed on thisarched surface, the socalled secondary surface. The known furnace isheated by conducting a current in the longitudinal direction through thetubular part of the furnace by means of existing contact members (notshown). When the furnace is being heated, the analyte is atomized andcondensed on the secondary surface.

When said secondary surface has reached the temperature of the furnaceatmosphere, the analyte is reatomized.

By means of an arrangement and a process of the above-mentioned type,the matrix effects occurring during atomization can be reducedsignificantly in view of the redistribution of the analyte. Furthermore,due to the condensation of the analyte on the secondary surface, thereatomization of the analyte is essentially based on desorption fromsaid surface. It follows that this atomization is essentiallyindependent of the original composition of the sample.

SUMMARY OF THE INVENTION

It is the object of the present invention to improve the known deviceand the known process in such a way that improved examination resultsare obtained.

For a device of the type referred to at the beginning, this object isachieved by the feature that the the furnace comprises a first sectionand a second section, which are adapted to be electrically heatedindependently of one another, the secondary surface being defined by asurface of said first section.

This permits a separate, precise temperature control of the two sectionsof the furnace, said temperature control being possible with regard tothe heating rates as well as with regard to the respective finaltemperatures. The heating program can be adapted to the analyticalrequirements in the best possible manner in this way.

In particular, this structural design permits a much better control ofthe temperatures of the gaseous phase and of the surfaces of theatomizer. Due to the fact that the secondary surface is controlledseparately, the time delay in the heating of the atomization surface canbe controlled independently of the aimed-at final temperature.

Furthermore, in contrast to the passive function of the secondarysurface in the prior art, the temperature difference between the twosections can be controlled with the aid of the separate control.

In addition, a control of the heating rates is possible due to the factthat the first and the second sections can be heated separately. Thenecessity of a construction-dependent compromise between the heatingrate and the delay--which compromise has to be made in the case ofsystems with platforms--no longer exists in the case of this system. Inparticular when the atomization of the analyte takes place from thesecondary surface, this will also permit increased heating rates and,consequently, an improved sensitivity in the case of reduced gaseousphase interferences.

According to a preferred embodiment of the present invention, thesecondary surface is a section of the inner wall of the hollow-bodyfurnace.

This has the additional advantage that the atomization means can beproduced with little effort.

In accordance with a preferred embodiment, the first and the secondsection can be provided with contact members by means of which electriccurrent can be supplied to said hollow-body furnace.

For this purpose, a common contact element can be provided for bothsections and two contact members which are electrically insulated fromeach other can be provided for said two sections.

Such an arrangement has the advantage that the atomization means can beused in known atomic absorption spectrometers without any necessity ofmodifying said known atomic absorption spectrometers with the exceptionof the current supply means and the control unit for the current supplymeans.

The material used for the hollow-body furnace is preferably graphite.This material guarantees that the furnace can be heated, by supplyingcurrent thereto, directly and with a very short response behaviour.

According to a special embodiment, the hollow-body furnace can have atubular structural design. In this connection, it will be particularlysuitable when the two sections have the shape of a semitube. These twosemitubular sections can then be assembled so as to form a tubular body,the contact members of the two semitubes being insulated from each otheron at least one side of said tubular body.

According to a special embodiment, the furnace tube of a transverselyheated tubular furnace is constructed such that it is slotted in thelongitudinal direction on one contact side, the slot defined in this wayis provided with insulating material and the walls of the tubularfurnace located on both sides of said slot are connected to separateelectric contacts.

An appropriate insulator for this purpose are pyrolytic graphite, theanisotropy of this material being used in a suitable manner, or ceramicmaterials.

These special embodiments permit the hollow-body furnace to be producedat a low price. Furthermore, this type of embodiment of the furnace caneasily be used in known atomic absorption spectrometers.

According to a further special embodiment, a dosing aperture can beprovided in the first section of the furnace. By means of this aperture,the sample can be applied to the surface located opposite the secondarysurface.

In this way, the analyte is distributed on the secondary surface in thebest possible manner for the purpose of atomization.

According to a further embodiment of the invention, the processmentioned at the beginning can be carried out such that a hollow-bodyfurnace is provided, which comprises a first section and a secondsection, which are adapted to be electrically heated independently ofone another, the secondary surface being defined by a surface of saidfirst section, that, after the application of the analyte to a surfaceof said second section, said surface is electrically heated so that atransfer of the analyte via the gaseous phase to the secondary surfacetakes place, said secondary surface being maintained at a temperaturewhich permits the analyte to condense on said secondary surface, andthat, after said transfer, the first section is electrically heated to atemperature corresponding essentially to that of the second section,whereby the analyte condensed on said secondary surface is desorbed andatomized.

An essential feature of this process is that the secondary surface ofthe first section and the second section are heated independently of oneanother. This has the effect that the time delay in the heating of theatomization surface, the control of the temperature difference betweenthe surface to which the analyte is applied and the secondary surfaceonto which said analyte is to be condensed as well as the heating ratecan be controlled precisely. In this way, the atomization process can beadapted to the given properties of the analytes to be detected muchbetter than in the case of known processes. The matrix effects duringatomization will be reduced in this way and, on the whole, a highersensitivity in the case of reduced gaseous phase interferences will beobtained. It follows that, taking all this into account, smaller amountsof analyte can be detected by means of this process.

According to a special embodiment of the present process, the analyte,after having been applied to the second section, is dried by heatingboth sections to essentially the same temperature. When this heating iscarried out synchronously and with the same temperature, a condensationof solvents or of coarse constituents on the surfaces of the atomizercan be avoided. This will especially improve the results when analytes,which are constituent parts of samples containing such components, areto be detected.

According to a further embodiment, the analyte, after having beenapplied to the second section, is dried by heating the first section toa temperature that is higher than the temperature of the second sectionso that the first section is used as a surface radiator. This permits anevaporation of samples which, when heated directly, evaporateirregularly or cause analyte losses. This process step reduces theanalyte losses in the case of such samples, and, consequently, moreprecise measurement results will be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the present invention will be explained in detail onthe basis of preferred embodiments shown in the drawing, in which:

FIG. 1 shows a cross-section through an atomization means according to afirst embodiment of the present invention;

FIG. 2 shows a cross-section through an atomization means according to asecond embodiment of the present invention;

FIG. 3 shows a diagram for explaining a process according to anembodiment of the present invention; and

FIG. 4 shows a cross-section of an atomization means according to theprior art.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows, in a cross-sectional view, an electrically heatabletubular furnace designated by reference numeral 1. This tubular furnacecomprises a first section 10 and a second section 20. These twosections, which have a semitubular structural design according to FIG.1, are separated by insulators 30 and 30', respectively.

The first semitubular section 10 is followed by contact members 31 and31', which extend radially outwards from said section 10. The secondsection 20 is followed by contact members 32 and 32', respectively, inthe same way. These contact members 31 and 31' as well as 32 and 32' areconnected to separate current supply means (not shown) so that acurrent, which causes said sections 10 and 20 to be heated, can beconducted through said sections 10 and 20.

The current supply means for said two sections can be constructed suchthat they are programmable so that an automatic and reproducable controlwith regard to temperature, time and heating rates is possible.

Furthermore, various programmable gas inlets (not shown) can beprovided, said gas inlets permitting different gases to be conductedinto in the furnace during the steps of the heating program as well as aflow around the furnace. Such gas inlets are known e.g. fromEP-A-0350722.

The first section 10 additionally comprises a dosing aperture 40.Through this dosing aperture, a sample, which will be a liquid sample inmost cases, can be introduced in the furnace with the aid of a dosingcapillary or an automatic dosing device (not shown).

In the embodiment shown in FIG. 1, the current is conducted through thefurnace transversely to the longitudinal axis thereof; i.e. the furnaceis heated in the transverse direction. This has the effect thattemperature gradients over the length of the tubular atomizer areavoided. The contact members can especially have a structural design ofsuch a nature that the first section 10 (and the second section 20,respectively ) are heated as uniformly as possible. Special embodimentsfor avoiding such temperature gradients and for guaranteeing uniformheating are known e.g. from EP-B-0381948.

The sections 10 and 20 are produced from a conducting material, such asgraphite or tungsten. The material used as an insulator is especiallypyrolytic graphite, the anisotropy of this material being used in asuitable manner. Alternatively, it is also possible to use ceramicmaterials as insulators.

In FIG. 2, a further embodiment of the furnace 100 according to thepresent invention is shown. In contrast to FIG. 1, the first section 110and the second section 120 have a common contact member 133. On the sidelocated opposite said contact member, two contact members 131 and 132,respectively, are provided for the first section 110 and the secondsection 120, in a manner corresponding to the embodiment according toFIG. 1. In the embodiment according to FIG. 2, it suffices to provide asingle current supply from the left side. As for the rest, the furnacecorresponds to the furnace shown in FIG. 1.

In addition to the tubular furnaces which are shown in FIG. 1 and 2 andwhich comprise said sections 10 and 20, furnaces having a differentstructural design can be used as well. On the one hand, two sectionshaving an arbitrary shape can be used, which, when assembled, define atubular furnace body and can be heated independently of one another. Onthe other hand, it is also possible to use hollow bodies having someother shape.

FIG. 3 shows a diagram for explaining a process for atomizing an analyteof a sample to be examined, said process being adapted to be used in afurnace according to FIG. 1 or 2.

In this diagram, the time is shown in the horizontal direction and thetemperature in the vertical direction. The time axis is subdivided intofour areas I, II, III and IV. These areas represent various processsteps, viz. drying, reduction to ashes, redistribution and atomizationin the process according to the present invention.

For each of these process steps, the temperature (solid arrows 210) ofthe first section 10 and the temperature (broken arrows 220) of thesecond section 20 are shown. Furthermore, the furnace is schematicallyshown in the form of its two sections 10 and 20 for each area, the flowof current through the first section 10 and through the second section20 being schematically outlined for each area by means of arrows 310 and320. The thickness of the arrows 310 and 320 is representative of theflow of current through the respective area.

When the sample to be analyzed has been applied in a suitable manner tothe inner side of the second section 20 through the dosing aperture 40(e.g. by means of an automatic dosing device), the sample is dried byprogrammed heating of the first and second sections 10, 20 and pyrolyzedin step I. According to FIG. 3, the first section 10 is heated to aslightly higher temperature than the second section 20 for this purpose.In this case, the first section 10 acts as a surface radiator by meansof which samples can be evaporated which, when heated directly,evaporate irregularly or cause-analyte losses.

In addition to the drying process shown, it is also possible to useother heating rates, which are adapted to the respective analyte to bedetected. It is, for example, possible to heat both sectionssynchronously and with the same temperature so as to avoid acondensation of solvents or sample components on the surfaces of theatomizer.

After the heating, the temperatures of the first section and of thesecond section are maintained at a constant value until the drying phasehas been finished.

In step II, the sample is reduced to ashes. For this purpose, the firstsection 10 and the second section 20 are heated in a manner similar tothe heating carried out in step I, and then they are maintained at aconstant temperature until the pyrolysis has been finished.

In steps I and II, a gas can be conducted through the furnace so thatsubstances formed during the drying step and the pyrolyzing step aredischarged.

These two heating steps are followed by step III. In this step, thesample is thermally decomposed and the analyte to be detected isredistributed via the gaseous phase from said second section 20 to saidfirst section 10. For this purpose, the second section 20 is heated veryrapidly to a temperature that is suitable for atomization. Thetemperature of the first section 10 is either maintained at the formerpretreatment temperature or, as shown in FIG. 3, lowered to a lowertemperature. In view of the fact that the surface of section 10 is muchcolder, the analyte condenses on the secondary surface 50 of saidsection.

The temperature difference between the upper and the lower section canbe adapted to the properties of the analyte and of the sample matrix ina controlled manner. During step III, the flow of gas through thefurnace is interrupted so that, at the end of step III, the analyte isquantitatively deposited on the secondary surface 50 of the firstsection 10.

By means of the transfer step III, the original sample is distributedover a much larger active surface and the original sample structure,which is e.g. a crystalline sample structure in the case referred to, isdestroyed. This has the consequence that the subsequent atomization ofthe analyte effected from the secondary surface 50 becomes independentof the original sample matrix to a very large extent.

Step III is followed by the actual atomization step IV. In this step,the first section 10 is heated to the aimed-at atomization temperatureas rapidly as possible. This has the effect that the analyte depositedon the secondary surface 50 of said section 10 desorbs from said surfaceand is transferred to the gaseous atmosphere preheated by the secondsection 20.

It goes without saying that also in this step the flow of gas can bereduced or interrupted in accordance with the sensitivity required.

In step IV, an atomic absorption spectroscopy measurement can be carriedout in the manner known.

When the measurement has been finished, the residual atomizationproducts are removed from the furnace by baking out. For this purpose,gas is again conducted through the furnace.

In the table following hereinbelow, a typical example of a control inaccordance with the process according to the present invention is given.

                  TABLE                                                           ______________________________________                                                temperature heating maintain-                                                                            gas   detec-                                 step (° C.) (s) ing (s) flow tion                                    ______________________________________                                        I:      section 20: 5 10    10     full  --                                     sample section 10:  250                                                       drying                                                                        II: section 20:  500 15  10 full --                                           reduction section 10:  500                                                    to ashes                                                                      III: section 20: 1800 0 0.5 -- --                                             redistri- section 10:  300                                                    bution                                                                        IV: section 20: 1800 0  3 -- active                                           atomiza- section 10: 1800                                                     tion                                                                          baking section 20: 2400 1  3 full --                                          out section 10: 2400                                                        ______________________________________                                    

We claim:
 1. An electrically heatable hollow-body furnace (1; 100)comprising:a first semi-cylindrical section (10; 110) having an innerwall and an outer wall and a second semi-cylindrical section (20; 120)having an inner wall and an outer wall, said sections positioned inclose relationship to form a tube, said sections designed for beingelectrically heated independently of one another; and, a secondarysurface (50) on which an analyte of a sample to be examined is condensedprior to being converted into the atomized state for examination, saidsecondary surface (50) being defined by a surface of said first section(10; 110).
 2. A furnace according to claim 1, wherein said secondarysurface (50) is a section of the inner wall of the hollow-body furnace(1).
 3. A furnace according to claim 1, wherein said first section (10;110) and said second section (20; 120) are connected to contact members(31, 31', 32, 32'; 131, 132, 133) for supplying electric current to saidhollow-body furnace (1; 100).
 4. A furnace according to claim 3, whereina common contact member (133) is provided for both sections (110, 120)and wherein two contact members (131, 132) which are electricallyinsulated from each other are provided for the two sections (110, 120).5. A furnace according to claim 1, wherein the hollow-body furnace (1;100) is comprised of graphite.
 6. A furnace according to claim 1,wherein both sections are separated by at least one insulator (30; 130)so that they are heated independently of one another.
 7. A furnaceaccording to claim 6, wherein said at least one insulator (30) comprisespyrolytic graphite or ceramic materials.
 8. A furnace according to claim6, wherein the tubular furnace is designed for being heated transverselyto the longitudinal direction thereof.
 9. A furnace according to claim1, wherein the first section includes a dosing aperture (40).
 10. Afurnace according to claim 1, wherein said first semi-cylindricalsection and said second semi-cylindrical section form a singlecylindrical tube.